The Police Service of Northern Ireland said 29-year-old journalist and author Lyra McKee was shot and killed, probably by a stray bullet, during rioting in the city's Creggan neighborhood. It said the dissident group, the New IRA, was most likely responsible.

Assistant Chief Constable Mark Hamilton said a gunman fired a number of shots at police during the unrest Thursday evening.

"We believe this to be a terrorist act," he said. "We believe it has been carried out by violent dissident republicans."

Hamilton said the force's assessment "is that the New IRA are most likely to be the ones behind this and that forms our primary line of inquiry."

McKee was a rising talent who had written eloquently about the challenges of coming out as gay in Northern Ireland. Her partner, Sara Canning, told a vigil that McKee's amazing potential had been snuffed out.

"It has left so many friends without their confidante," she said.

Canning said the senseless murder "has left me without the love of my life, the woman I was planning to grow old with."

A murder investigation has been launched but there have been no arrests yet. Police appealed for calm to prevail over the long Easter holiday weekend.

Speaking in Dublin, Irish Prime Minister Leo Varadkar said the people of Ireland and Northern Ireland had chosen peace and cooperation 21 years ago and will not be "dragged into the past" by political violence.

The New IRA is a small group who reject the 1998 Good Friday agreement that marked the Irish Republican Army's embrace of a political solution to the long-running violence known as "The Troubles" that claimed more than 3,700 lives.

The group is also blamed for a Londonderry car bombing that did not cause any injuries in January. It is regarded as the largest of the splinter dissident groups still operating and has been linked to several other killings in the past decade.

Deputy Chief Constable Stephen Martin said Friday that police believe more than one person was involved in the shooting.

"We certainly believe there was more than one person who was involved in this last night. Obviously only one person pulled the trigger but there was more than one person," he said.

He said the violence started after police entered the area to search for weapons and that the gunman was aiming at policemen when the rioting intensified.

"The full and total responsibility for Lyra McKee's death lies with the organization that sent someone out with a gun," he said.

Londonderry Mayor John Boyle said the city was united in mourning McKee's death.

"I have known her since she was 16 years old," he said. "She was bright, she was warm, she was witty. But most of all she was an outstanding individual, a great friend to so, so many people in this city in the short time that she was with us."

There has been an increase in tensions in Northern Ireland in recent months with sporadic violence, much of it focused in Londonderry, also known as Derry.

McKee rose to prominence in 2014 with a moving blog post — "Letter to my 14 year old self " — describing the struggle of growing up gay in Belfast, the capital of Northern Ireland.

In the post, she described the shame she felt at 14 as she kept the "secret" of being gay from her family and friends and the love she eventually received when she was finally able to reveal it.

McKee had recently signed a contract to write two books.

Hours before her death, she tweeted a photo of the riot with the words: "Derry tonight. Absolute madness."